In practice a plurality of optical alarm systems for area protection have been known and used, the common characteristic of which lies in that in the majority of the cases infrared light is used in the established light barriers. Said light barriers radiate modulated or unmodulated light in a continuous mode of operation, accordingly, transmitters and receivers of the light barriers require power of such a magnitude, which cannot be supplied economically either from a battery. For this reason the required number of transmitters and receivers of the light barriers are arranged in a stationary way, simultaneously power is provided for, representing--as widely known--the weak point of the protection systems.
Due to the aforemention features known optical alarm systems for area protection are not at all suitable for a changeable settlement or as portable embodiments, not in the least degree for battery operation, although there is an ever increasing demand in this respect.